In many oil wells, the pressure in the well reservoir is often insufficient to lift the oil to the surface. In such cases, it is conventional to use a sub-surface pump to force the oil out of the well. The sub-surface pump is driven by a pumping unit located at the surface. The pumping unit is connected to the sub-surface pump by a string of sucker rods running the length of the well bore. The pumping unit moves the sucker rod string up and down in the well bore to drive the sub-surface pump.
For many years sucker rods were generally made of steel. Due to the heavy weight of the steel rods, large pumping units were required and pumping depths were limited. It is now preferable to use sucker rods made of fiberglass or composite material with steel connectors joining the rods together to make a string of the required length. Fiberglass rods provide sufficient strength to tolerate the mechanical stresses of pumping, and yet weigh substantially less than steel rods. Another advantage of fiberglass or composite sucker rods ("FSR") over steel is their improved resistance to the chemical stresses encountered in corrosive environments. Fiberglass rods have been used successfully in the field since 1973, and have proven to be of particular value in corrosive environments where steel rods have an unacceptable failure rate due to weakening of the steel from corrosion and high load levels.
Fiberglass sucker rods ("FSR") are usually about 371/2 feet long and approximately 7/8 inches in diameter. Each rod is composed of bundles of glass filaments (rovings) approximately 15 microns in diameter that have been wetted with a resin and formed into a rod. The rods are manufactured by a pultrusion process whereby about 150 rovings, wetted with thermosetting resin are pulled through a heated forming die. The heat catalyzes a chemical reaction causing the resin to harden and bonding the rovings and the resin together into a composite solid which is formed into a rod by the die. It is critical that the rods be manufactured so as to prevent looping of the rovings or other imperfections which introduce flaws in the rod body greatly increasing the odds of rod failure in the field.
Sucker rods are connected together in a string by steel connectors attached to the ends of each rod. With the solving of rod manufacturing problems such as looping, the steel connectors or end fittings between rods have proven to be the source of most composite rod failures or end fitting pullouts. Therefore, the sucker rod connectors have been the focus of recent efforts to improve the reliability of fiberglass or composite sucker rod construction.
The end fittings comprise a rod receptacle at one end to receive the rod end, and a threaded coupling at the other end to threadedly connect to the end fitting of the next successive rod. The space between the interior wall of the rod receptacle and the external surface of the rod defines a space or annulus which is filled with epoxy or some other initially flowable adhesive such as epoxy. The epoxy cures into a solid which bonds to the rod. Typically, the adhesive is heat activated and heat is applied to the rod as a curing agent. Early experiments with such connectors resulted in rod pullouts, where the rod is pulled out of the connector rod receptacle causing failure of the string. Such string failure can be catastrophic, requiring expensive repairs or even well closure.
Current end fittings are formed such that the epoxy cures into a series of wedges that cooperatively engage complimentary surfaces in the rod receptacle to prevent rod pullouts.
FSRs were developed to improve the operation characteristics of artificial lift rod pumping systems in crude oil production.
The use of FSR in rod pumping systems is indicated when analysis of the down hole pumping system(s) reveals a need for the particular performance characteristics offered by FSRs, which characteristics comprise resistance to corrosion, light rod string weight, lower pumping unit gearbox loads, and the "rubber band" effect due to the elastic properties and geometric shape memory after elongation of the fiberglass (or composite) component of the system. Fiberglass sucker rod pumping systems have become an accepted ingredient in artificial lift design, and are used extensively throughout the range of crude oil production.
Among the mechanical forces acting on the rod/adhesive/metal interface, are compressive forces, such as during a stroke of the pump either up or down, and negative load forces. Negative load refers to forces acting on the side of the wedge opposite from the gripping side of the wedge. Negative load is very destructive to the wedges of prior art designs, causing catastrophic shear failure of the wedge. In the present invention, however, when a shock load occurs that creates a negative load, the wedge has the ability to absorb the negative load forces and to thereby resist failure of the rod connection.
Early rod designs were plagued with early time to first failure. Failure analysis of early FSR designs revealed the following:
A. Failure, while exhibiting itself catastrophically, is rarely a result of a catastrophic evens. The exhibition of catastrophic failure is usually a result of improper maintenance and materials handling procedures. PA1 B. Failure, regardless of its manifestation, can be linked to the interface between the fiberglass rod and the metal end fitting. PA1 C. End fitting designs that distribute applied stresses more fully along the length of the interface are more successful in reducing failure. PA1 A. Pinch-off--wherein the fiberglass/adhesive/metal interface is abruptly sheared, and the rod is sheared away from the end fitting; and PA1 B. Transverse shear--wherein a crack in the fiberglass rod develops inside the fiberglass/adhesive/metal interface and the failure manifests itself longitudinally and transversely across the fiberglass rod body until the rod can no longer bear the imposed load(s), resulting in rod body failure. PA1 A. Shear forces spike into the fiberglass rod locally to the is discontinuity of the metal; the shear forces develop perpendicular to the diameter of the glass fibers and the resultant shear beaks the glass fibers causing failure. PA1 B. Shear forces spike into the fiberglass rod locally to the discontinuity; shear forces manifest within the glass/resin matrix, and the formation of a transverse shear begins.
The design of the metal end fitting has consistently comprised a wedge shaped pocket (receptacle) to accept the fiberglass rod. The following procedure applies to various diameters of rod sizes, and the principles and practices remain the same regardless of rod size. Current production practices involve the preheating of an end fitting, filling the end fitting with a one part heat activated adhesive, installing an end fitting onto both ends of a fiberglass rod of some length, and heating the area(s) to include all of the interface between the metal and fiberglass. It is important that in such a system, the adhesive layer serves to adhere to the fiberglass only, and not the end fitting pocket. The adhesive layer thus acts as a plug being wedged by force to the end fitting pocket socket. After proper time intervals and heat application, the assembly is then tested by application of force directed coaxially in opposing directions to test the wedge strength and to "set" the end fitting wedge receptacle with the hardened adhesive. The pocket or pockets in the end fitting serve as both the mold to form the wedge or wedges from the fluid adhesive, and as receptacles to capture the hardened adhesive wedges.
Wedges transmit the compressive and tensing forces of pumping from the steel connector to the fiberglass rod and vice-versa. The metal end fitting is harder than the hardened adhesive, and deforms the shape of the hardened adhesive wedge. Essentially, the metal end fitting squeezes the deformations in the adhesive when compressive and back travel forces are applied to the construction. Ideally, the deformations are squeezed by the end fitting out toward the end of the rod, transmitting the forces, at least to some extent, into the metal end fitting for optimum dispersal of destructive forces.
Axial forces applied to a rod cause deformations of the rod material. The deformations are transmitted throughout the rod body and vary depending on the magnitude of the force and the cross-sectional area of the rod. Abrupt changes in the cross-sectional area of the rod concentrate stress forces in certain areas of the rod. The wedges of sucker rod connections change the cross-sectional area of the rod in comparison to the rod body in such a way as to concentrate stress forces on the rod. The concentrated forces may exceed the structural strength of the composite material of the rod, resulting in rod failure from cracking or splintering.
Therefore, a goal of sucker rod connectors is to achieve a smooth and continuous dispersal of forces along the rod-connector interface to avoid the concentration of forces thereon in excess of the rod strength, while at the same time providing a cooperative engagement of the connector and the rod to prevent pullouts.
In order to make the attachment of the steel end fitting to the fiberglass rod, an initially flowable adhesive is placed in the receptacle of the connector. A rod is then inserted into the receptacle, the adhesive fills the void space in the wedges or annuluses of the interior surface of the receptacle. The initially flowable adhesive cures or hardens becoming a solid and adhering to the rod. The adhesive bonds to the rod and not to the inside of the metal receptacle.
When the assembled rod is pulled in tension in its connector, the solid adhesive wedges bonded to the rod press against the complimentary form of the interior of the end fitting and force the end fitting against the annular wedges of the solid adhesive. A compressive force is imparted to the rod itself as the metal connector and the adhesive wedge press against each other to resist any further slippage. This force of compression is applied across the entire surface where the adhesive wedge and the metal surface contact. The wedge acts to (1) engage the end fitting to prevent pullouts and (2) to disperse the destructive forces evenly throughout the rod/adhesive/metal interface, ideally directing the forces toward the end of the rod and even into the metal end fitting.
Experience has shown that any abrupt discontinuity in the angle of the wedges of the end fittings can result in the compressive forces being concentrated at the area of discontinuity. The force can exceed the strength of the fiberglass rod at the point of discontinuity, resulting in rod failure.
Failure of FSR in production are most often encountered in one of two scenarios:
Due to the concentration of applied forces, the imposed increase in stress is transferred from rod to end fitting, and conversely, end fitting to rod, in localized areas of insufficient area so as to absorb and/or distribute the applied forces. The resulting stress concentration is de-energized by one of two methods:
The contours of the wedges on the interior surface of the end fitting affect the shape of the distortion in the shape of the adhesive material. The distortion travels through the adhesive, impelled by the mechanical stress and strain forces acting on the end fitting. Specifically, the shape of the distortion approximates the shape of the wedges. If the wedges have an abrupt change of cross sectional area such as a point of transition from one wedge to the next successive wedge, the shape of the abrupt change will be echoed in the shape of the distortion, with the result that the distortion takes on a "spiked" shape. The spike is a manifestation of the concentration of force caused by the abrupt discontinuity in the wedges. Such concentrated forces may exceed the material strength of the rod, particularly where the spike is impelled into the rod at the interface of the rod and the adhesive.
Inadequacies in the stress distribution dynamic lead to localized and intense stress risers that can overcome the properties of the rod/adhesive/metal interface to adequately distribute the applied load(s), resulting in the loss of integrity of the interface system. Additionally, the cumulative effect of repetitive stress risers aggravate the loss of integrity, thus accelerating the erosion of the affected area. Thus, any attempt to minimize the destructive forces leading to catastrophic failure must be focused on the fiberglass/adhesive/metal interface.
In any end fitting design, the principle of the wedge is employed to provide capture of the fiberglass rod and distribution of the applied forces encountered in field use. The wedge is formed by a rod receptacle having an interior surface shaped to form at least one generally wedge-shaped annulus between the interior surface of the receptacle and the end of the rod received by the receptacle. The wedge-shaped annulus has an annularly thin portion and an annularly thick portion distal to the thin portion.
Examples of end fitting designs include from five wedges (being the earliest designs) to one wedge. In each design, the shape (or shapes) of the wedge (or wedges) is/are determined by the diameter of the fiberglass rod, the diameter of the pocket (receptacle) of the end fitting, and the length of each wedge section. In all cases, areas of discontinuity and abrupt changes in the shape of the pocket lead to high stress levels, as revealed by stress analysis of the particular system. Examination of the stress distribution, or lack thereof, reveals that these areas of high stress concentration are a product of the shape and size of the discontinuity of the end fitting pocket. These areas lead to destruction of the rod/adhesive layer, leading to catastrophic failure as described above.
There is a need, therefore, for a sucker rod end fitting in which compressive forces are transmitted to the fiberglass rod without excessive concentration of compressive forces in any portion of the rod.
There is a further need for a sucker rod end fitting wherein the internal wedges have no area of abrupt discontinuity.
Therefore, it is an object of the present invention to provide a sucker rod end fitting in which compressive forces are not excessive in any portion of the rod.
Another object of the present invention is to provide a connector for connecting rods end to end, wherein the connector distributes stress forces acting on the rods from the connector equally across the diameter of the rods.
It is a further object of the present invention to provide a sucker rod end fitting wherein the transition from one wedge to the next contains no abrupt discontinuities. That is, the transition from one tapered annulus to the next tapered annulus is a continuous curve in the shape of a wave.